In the above-mentioned manual surveying instrument, a collimating operation is necessary at the time of measurement so that a target such as a prism is located on a collimation axis. The collimating operation is to turn the collimating telescope around so as to be trained upon the target, capture the target within a field of view of the collimating telescope, and align, by means of a coarse and fine motion mechanism provided in the surveying instrument, the target with the center (collimation axis) of crosshairs observed within the field of view of the collimating telescope.
Meanwhile, in the above-mentioned manual surveying instrument, collimation errors according to observer proficiency have existed, and in the case of an inexperienced observer, there has been a problem that the observer can perform only highly erroneous measurements even by use of a valuable high-precision surveying instrument. Moreover, there has also been a problem that, in an atmosphere with much shimmering such as heat haze, it is difficult even for an experienced observer to perform an accurate collimation, and the observer can perform only highly erroneous measurements.
In order to solve such problems, there has been proposed, in the following patent document 1, a surveying instrument that allows obtaining an accurate angle measurement value even when collimation of the target is somewhat imperfect. This surveying instrument will be described based on FIG. 4.
A collimating telescope 3 of this surveying instrument includes an objective lens 11, a focusing lens 14, a focusing glass 15, and an eyepiece 12 on an collimation axis O. There are crosshairs drawn on the focusing glass 15, so that an observer can perform manual collimation looking through the eyepiece 12. Moreover, this surveying instrument includes, in order to transmit an infrared laser light P along the collimation axis O, a laser diode 17, a condenser lens 18, a relay lens 19, a beam splitter 20, and a dichroic mirror 13. Further, this surveying instrument includes, in order to receive a laser light reflected on a target 23 such as a prism, the dichroic mirror 13, the beam splitter 20, a filter 21, and a light receiving element 22 such as a CCD area sensor.
When the target 23 is accurately located on the collimation axis O, the laser light P reflected on the target 23 returns along the collimation axis O, and the target 23 is imaged on a center C of the light receiving element 22. At this time, an angle measurement value (vertical angle, horizontal angle) measured by an angle measuring unit (not shown) is accurate.
On the other hand, when the target 23 has not been able to be accurately collimated, and the direction of the target 23 has a horizontal angle deviation ΔH from the direction of the collimation axis O, an angle measurement value measured by the angle measuring unit is inaccurate. However, based on a horizontal direction deviation Δh from the center C of the light receiving element 22 being the imaging position of the target 23 on the light receiving element 22, the horizontal angle deviation ΔH can be calculated. Then, by applying to a horizontal angle H obtained from the angle measuring unit an addition subtraction correction through substituting the horizontal angle deviation ΔH for a predetermined function, an accurate horizontal angle can be obtained. An accurate vertical angle can also be obtained in the same manner.
Therefore, this surveying instrument provides an effect of allowing obtaining an accurate angle measurement value even with somewhat imperfect collimation, and also allowing shortening the time required for measurement.
As an example of another known art, there has also been disclosed, in the following patent document 2, an instrument for which a collimating telescope and a wide-angle digital camera are provided in an automatic collimation device, and which first captures a target in the vicinity of a collimation axis by the wide-angle digital camera, and then performs switching to an imaging device (area sensor) and the collimating telescope to accurately automatically collimate the target.    Patent Document 1: Japanese Patent No. 3748112    Patent Document 2: Japanese Published Unexamined Patent Application No. 2003-279351